ROSEN, STUDIES ON THE PLECTOGNATHS. 17 



tbe skin is very thick but owing to the arrangement of the 

 connective tissue fibres being not in straigbt layers these 

 allow a great extension. As will be shown in the sectionon 

 the integument only the exteriör of the corium contains some 

 but still comparatively rather few, elastic fibres, The epithe- 

 lium is folded as shown in fig. 1 Pl. I, but the folds are 

 smoothed out when the skin is extended. Another direct 

 adaptation for producing inflation very extensive is the great 

 subcutaneous stratum of loose connective tissue that unites 

 the air-sac with the ventral body wall in the Tetrodontids. It 

 forms, we may say, a sort of packing between the two walls, 

 that of the air-sac and the cutis, making the smoothing out 

 of their folds easier. In many of the Balistids the body is 

 covered with large scales, forming a compact mantle which 

 would not allow of any extension. In several species of 

 Monacanthus this begins to become reduced. In those which 

 have a moveable skin flap behind the ventral spine, the scales 

 are on this very feeble. In the Tetrodontids and Diodontids 

 there is no continuous layer of scales but spines separated 

 from each other. As mentioned above these spines become 

 erected when the body is inflated, forming an effective means 

 of defence. 



I have not yet studied the musdes of the Plectognaths 

 sufficiently to be able to state to what extent they are in- 

 fluenced by the inflation of the body. I have, however, in 

 the Tetrodontids and Diodontids found a direct adaptation 

 of the ventral body muscles in order to contract the belly 

 thus expelHng the air. These muscles are, as above mentioned, 

 arranged in an exteriör transverse layer, and an interiör 

 longitudinal one. Perhaps a closer study of the muscles of 

 the head will show an enlargement of some of them, as without 

 doubt a rather great power is necessary for thefilling of the 

 air-sac in the Diodontids and Tetrodontids. 



The inflation of the air-sac, when this has reached the 

 large size as in the Diodontids and the Tetrodontids, must 

 put a pressure on all ilie organs in the hodij cavity. As a direct 

 consequence thereof may be regarded the reduction of the 

 liver. In Balistes the liver consists of two large flaps united 

 broadly anteriorly in the median line. When the air-sac is in- 

 flated it extends mostly towards the direction of the least 

 resistence, viz. the ventral side. If these fishes had possessed 



Ärhiv för zoologi. Band 7. X:o 30. 2 



